1. Field of the Invention
The present invention relates to a resin-molded semiconductor device and, more particularly, to a construction of a resin-molded semiconductor device attachable directly to an external heat radiating fin.
2. Description of the Prior Art
In a typical semiconductor device in the prior art, a semiconductor pellet having a transistor formed thereon is attached to a predetermined portion of a metallic heat radiating plate. A collector lead is secured to the metallic heat radiating plate by, for example, welding, while the emitter and base leads are arranged such that they are positioned near the metallic heat radiating plate. The emitter and the base electrodes of the semiconductor pellet are connected to the emitter and base leads by bonding fine metallic wires. Then, the vicinity of the heat radiating plate including tips of the emitter and base leads are covered by a molded resin. In the production of this type of resin-molded semiconductor device, the back surface of the metallic heat radiating plate opposite to the surface on which the semiconductor pellet is attached is held in contact with the bottom of the mold during the resin molding process, so that the back surface of the metallic heat radiating plate is exposed through the molded resin in the final product.
Therefore, when this resin-molded semiconductor device is attached to an external heat radiating fin, it is necessary to interpose a thin insulating film between the exposed back surface of the metallic heat radiating plate and the external heat radiating fin. In the recent years, to obviate this trouble, a resin-molded semiconductor device has been devised such that the whole surface of the heat radiating plate is covered with the molded resin to permit a direct attaching of the semiconductor device to the external heat radiating fin.
The production of such a resin-molded semiconductor device, however, encounters a difficulty in keeping the heat radiating plate afloat in the mold cavity to cover the back surface of the heat radiating plate with the resin. To this end, the end of the heat radiating plate opposite to the end to which the collector lead is connected is thinned as much as possible and made to project over a small width, and the resin molding is conducted while cantilevering the heat radiating plate at the projected end by means of the mold. Thus, the heat radiating plate partially projects out of the molded resin in the final product. Therefore, when this resin-molded semiconductor device is mounted on the external heat radiating fin, an atmospheric discharge tends to take place between the heat radiating fin and the projected portion of the metallic heat radiating plate.
According to another known method of producing this type of resin-molded semiconductor device, the molding is conducted while supporting the metallic heat radiating plate afloat by means of pins which are projected into the mold cavity. In this case, holes are left in the molded resin after the withdrawal of the supporting pins, and the heat radiating plate is exposed through the holes. Consequently, atmospheric discharge tends to be caused between the exposed portions of the metallic heat radiating plate and the external heat radiating fin.
In order to prevent the atmospheric discharge, it has been proposed to apply an insulating paint to the projected or exposed portions of the heat radiating plate. The insulating paint, however, cannot provide sufficient dielectric strength and, hence, cannot prevent the atmospheric discharge perfectly.